1. Field of the Invention
The invention relates to a cam drive for a temperature-sensitive device, in particular for a diaphragm gas meter, with
a part rotatable about an axis,
at least one cam element, which is unrotatably connected to the rotatable part,
at least one swivel arm which is pivot-mounted on the cam element,
a transmitting or sensing element which is connected to the swivel arm and
a temperature-dependent compensation element, which determines the angular position of the swivel arm.
2. Description of the Prior Art
Diaphragm gas meters have at least one chamber which is divided into two by a moveable diaphragm. The two chambers are connected alternately to a gas inlet and a gas outlet. Cyclical changeover is performed by a slide valve system whose movements backwards and forwards are initiated by the diaphragm, namely by the activation of the cam drive, the cam element being driven by the diaphragm whilst the slide valve system is driven by the rotatable part.
Generally, the rotatable part is designed as a shaft whilst the transmitting or sensing element is a crank pin. Cam drives of different designs are also conceivable. For example, the rotatable part can be a simple disk whilst the sensing element can be designed as a boring in which a bolt engages.
The cam drive has an extremely wide field of use. The invention will, however, be described as used in a diaphragm gas meter since this is considered to be the preferred use of the invention.
Diaphragm gas meters are temperature-sensitive, above all with regard to dependence of the gas density on temperature (3K corresponding to a volume change of approx. 1%). The gas is supplied in buried gas pipes and is therefore substantially at a constant temperature. If the room in which the diaphragm gas meter is installed also has a substantially constant temperature, obviously no problems occur. However, there are problems when the diaphragm gas meter is installed on the outside wall of a building and subjected to temperature fluctuations which may easily be 60 to 70K. Here the temperature-dependent compensation element is activated which determines the angular position of the swivel arm.
From practice it is known to arrange the swivel arm such that the compensation element adjusts the sensing element substantially radially to the axis of the rotatable part. This causes the stroke of the diaphragm of the diaphragm gas meter to change. The volume which can pass through the meter per counting unit therefore adjusts to the prevailing outdoor temperature.
It has been found that temperature compensation of the cam drive is still in need of improvement. Therefore, it is an object of the invention to improve the temperature compensation of the cam drive.
To achieve this object, the aforementioned cam drive is characterised in that the transmitting or sensing element can be adjusted relative to the swivel arm by a control means which is adapted to the type-specific temperature dependence of the particular temperature-sensitive device.
The invention is based on the fact that the temperature-dependence of the device, for example the diaphragm gas meter, is type-specific. This was previously not allowed for in temperature compensation. Therefore, there were different error curves for different types of meter which meant that a temperature compensation feature which was adjusted to suit a certain meter type and was sufficiently accurate for such a meter type produced inadmissible error curve deviations with other meter types.
By contrast, in the present invention the control means is adapted to the type-specific temperature dependence of the device. The swivel arm still carries out a temperature-dependent adjustment of the cam stroke as a function of the compensation element. This adjustment is superposed by the intervention of the control means which adjusts the sensing element relative to the swivel arm. Therefore, the error curve can be optimised for each type of meter using the control means.
Above all, it is possible to adjust the sensing element not only radially but also tangentially to the axis of the rotatable part. Thus the offset angle of the sensing element to the cam element and thus to the rotatable part changes. In use in the diaphragm gas meter this means that the advance of the slide valve control, related to the stroke movement of the diaphragm, is changed. While a radial adjustment of the sensing element through the stroke of the diaphragm causes a shift in the entire error curve in the direction of the zero line, a tangential adjustment leads to a flattening of the error curve. Therefore, a type-specific optimisation of temperature compensation can be achieved.
One embodiment of the invention proposes that the control means is linked to the temperature-dependent compensation element as the latter already represents a temperature sensor, the control means preferably being linked to the compensation element via the swivel arm of the sensing element, whose angular position is determined by the compensation element and thus represents a temperature-dependent value.
A particularly advantageous embodiment is characterised in that the control means exhibits a coulisse or recess in the cam element, in which recess the sensing element is guided. The simplest way of setting the characteristics of this temperature compensation feature is the shape of the recess. If the compensation element performs a swivelling of the swivel arm, the sensing element is adjusted simultaneously relative to the swivel arm. Cam stroke and advance are therefore adapted to the relevant temperature by one and the same activation of the compensation element.
Particularly favourable conditions are achieved by the fact that the sensing element can be adjusted relative to the swivel arm in the direction of the pivot point of said swivel arm.
The pivot point of the swivel arm can be arranged at any point on the cam element. The shifting of the sensing element superposed with the swivel movement of the swivel arm allows any design of the recess.
It is particularly advantageous to arrange the pivot point of the swivel arm eccentrically to the axis of the rotatable part on the cam element. The swivel arm can, under these circumstances, be aligned substantially tangentially to the cam element so that the forces introduced into the sensing element can be transferred via the guide walls of the recess to the cam element without substantial loading of the swivel arm.
Given this aspect, it is particularly advantageous for the line between the sensing element and the pivot point of the swivel arm to form a substantially right angle with the line between the sensing element and the axis of the rotatable part.
One embodiment of the invention proposes that the temperature-dependent compensation element is supported on the cam element and exhibits an actuating arm acting in an articulated manner on the swivel arm, said actuating arm being directed substantially away from the pivot point of the swivel arm. The transmission ratio between the actuating arm of the compensation element and the swivel arm can in this manner be very easily preselected as it depends on the distance between the pivot point of the swivel arm and the engagement point of the actuating arm.
It is advantageous for the actuating arm of the compensation element to be designed as a free end of a spiral-shaped bimetal element.
The invention is explained in more detail in the following with the aid of a preferred embodiment and the attached drawing.